Marine and land-based industrial (M & I) gas turbine engines are frequently derived from engines designed for and used in various types of aircraft. Such M & I gas turbine engines are used, for example, for powering marine vessels, electrical generators, and various types of pumps.
The parent gas turbine engine of an M & I engine is typically designed and constructed to be lightweight and to operate at minimum specific fuel consumption (SFC) in an aircraft for predetermined thermodynamic cycles of operation having predetermined ranges of air and combustion gas flow rates, temperature, and pressure in the engine.
Development of an aircraft gas turbine engine requires a substantial amount of design, development, and testing resulting in substantial development costs. In designing gas turbine engines for marine and industrial applications, it has proven to be more cost effective to modify an existing aircraft gas turbine engine in the desired power class, than to design the M & I engine from the beginning. Accordingly, it is desirable to minimize the changes in the aircraft engine required for obtaining a suitable M & I engine. One application of an M & I engine is to provide output shaft horsepower for powering an electrical generator to provide electrical power to a utility electrical power grid, or network, for either meeting base load demands, or peaking demands greater than the base load. One goal in providing electrical power is to generate the required power during base load or peaking operation as efficiently as possible for reducing kilowatt-hour costs.
One factor in obtaining relatively low kilowatt-hour costs is the development cost for providing an industrial gas turbine engine for meeting the required power demands. In order to keep development costs relatively low, the industrial gas turbine engine typically utilizes the parent aircraft gas turbine engine and makes as few changes in the design thereof as practical for obtaining the desired land-based gas turbine engine. Accordingly, the parent aircraft gas turbine engine utilized for M & I applications may be adapted specifically for particular applications, including, for example, driving an electrical generator at a synchronous speed, such as 3000 rpm or 3600 rpm, for generating electricity at 50 Hertz or 60 Hertz, respectively.
One type of gas turbine engine used for powering an electrical generator includes two rotors. More specifically, the engine includes in serial flow relationship a conventional booster compressor, core engine, and power turbine having an output shaft connectable to the electrical generator. The power turbine also includes a first shaft, or rotor, joined to the booster compressor, and the core engine includes a conventional high-pressure compressor (HPC) joined to a conventional high-pressure turbine (HPT) by a second shaft, or rotor. The first and second shafts rotate independently of each other but are predeterminedly controlled for conventionally matching fluid flow rates between the booster compressor and the core engine, for example. Such an industrial gas turbine engine may be conventionally derived from an aircraft gas turbine engine by eliminating the conventional fan disposed upstream of the booster compressor in the aircraft gas turbine engine, and modifying the booster compressor, for example by modifying the first few rotor stages thereof as is conventionally known for use in powering an electrical generator. Downstream of the power turbine, a conventional industrial exhaust assembly is provided for discharging the combustion gases from the power turbine to the atmosphere.
In operation for powering an electrical generator, the industrial engine must be operated first for bringing the rotational speed of the power turbine and output shaft to the required synchronous speed, for example 3600 rpm, before the generator may be electrically connected or locked-on to the electrical power grid which will then conventionally maintain the generator and the power turbine at that speed, i.e. synchronous speed. However, the parent engine was initially designed for providing substantial horsepower from the power turbine at that synchronous speed for powering the fan, which was removed for this industrial application, which provided substantial thrust for powering the aircraft in which the engine was utilized.
For example, one such dual rotor industrial engine for powering an electrical generator is configured for obtaining about 56000 maximum shaft horsepower from the output shaft for operating an electrical generator at a full electrical power, on-line synchronous running condition. At part power, on-line synchronous running condition, the minimum output shaft horsepower at which the power turbine may operate in this exemplary embodiment, is about 6800 shaft horsepower. This relatively low value of output shaft horsepower is obtainable by closing to their fullest extent, i.e. about 40.degree. closed, conventional booster variable inlet guide vanes (VIGVS) disposed upstream of the booster compressor which controls inlet airflow to the booster compressor. And, conventional booster variable bleed valves (VBVs) disposed at the discharge of the booster compressor are positioned in a fully open position for bleeding overboard a portion of the air compressed by the booster compressor. In this way, the flow rate of the compressed airflow to the core engine is substantially reduced for minimizing the output shaft horsepower. However, at the 6800 output shaft horsepower condition, the electrical generator prior to being locked-on does not provide a corresponding amount of load for accommodating that amount of output shaft horsepower. Thusly, lock-on at the required synchronous speed cannot be obtained. Furthermore, without suitable means for allowing lock-off from the power grid, the power turbine would reach undesirable overspeed conditions.
Accordingly, the parent aircraft engine could be further modified by replacing the original VBVs with suitably larger VBVs for bleeding additional compressed air from the booster compressor, and the VIGVs may also be modified for closing even further the inlet to the booster compressor for further reducing airflow through the booster compressor. However, this is generally undesirable since it requires additional structural changes to the parent engine, and undesirable pressure and temperature distortions in the compressed airflow channeled to the core engine may be created.
In the exemplary engine described above, the required flow area of the VBVs would have to be increased twice as large as the original flow area for reducing the output shaft horsepower to a substantially zero value for allowing lock-on of the generator to the electrical grid. Substantially zero output shaft horsepower means that amount of horsepower required for overcoming windage losses and other loads of rotating the electrical generator rotor prior to the production of electrical power therefrom. And, of course, the power turbine also provides additional horsepower through the first rotor for powering the booster compressor.
An additional problem associated with operating an aircraft-derived industrial engine for powering an electrical generator occurs during lock-off of the electrical generator when it is electrically disconnected from the power grid. In this situation, the output power of the engine is reduced to a minimum value, and, when the electrical generator is disconnected or locked-off from the power grid, the electrical load on the power turbine is eliminated. If the output shaft horsepower from the power turbine is too large, the power turbine will immediately overspeed, resulting in undesirable booster compressor stall, for example. Accordingly, in the lock-off condition, means must be provided for reducing the horsepower from the output shaft to substantially zero for preventing overspeed of the power turbine and resulting stall of the booster compressor.
Furthermore, during a full power emergency stopcock, the low pressure rotor system including the power turbine and the electrical generator coasts down in speed very slowly relative to the core engine and can cause booster compressor stall. Thus, during unfired rolldown, it is desirable that the power turbine not extract power from the flow field so that the low pressure rotor system decelerates faster for reducing the possibility of booster stall.